It has been 20 years. I don't know where the nickel was stored. Exposure to different gases can cause this. Intentional exposure to heat sources could do something. I don't believe it carries any extra value, but if you like the way it looks you can always save it in an inexpensive 2x2 coin flip. There are probably dozens of reasons why or how a coin will tone. Albums, (the cardboard out gases), environmental factors. Natural and unnatural toning. It's hard to pin it down to one thing.
This is a case of, "If only that coin could talk".... As has been stated, there is no way to put your finger on the journey this nickel has taken during it's twenty years. I would guess some heat source. Maybe on the dashboard of somebody's car in the deep south summer heat. There's just no way to tell.
At the top of the coin, it appears that another coin was laying on top of it, see the curve where there is no toning. So that part of the coin didn't have the same exposure as the rest of the toning.
that actually looks like one of the coins I "cleaned" in a small plastic cup with grungy nickels, dimes, quarters and cents with acid. The top part I'm sure a coin would fit over nicely protecting it from the "wash" generated from the acid fallout. LOL
